The invention relates to a construction including a metal-oxide high Tc superconductor material, whose parts include at least a substrate made from an electrically insulating material, the coefficient of thermal expansion of which is matched to that of the superconductor material, an intermediate layer arranged on the substrate, at least one buffer layer which has been deposited on the intermediate layer and a layer of the superconductor material which has been deposited on the buffer layer. The invention also relates to a process for producing a corresponding superconductor construction. A construction of this general type and a corresponding production process are to be found in European Patent Specification 0,312,015 B.
Superconductive metal oxide compounds with high critical temperatures Tc above 77 K are known, and these compounds are therefore also known as high Tc superconductor materials or HTS materials and, in particular, enable a liquid nitrogen (LN2) cooling technique to be used. Such metal oxide compounds include, in particular, cuprates of special material systems, for example of the Yxe2x80x94Baxe2x80x94Cuxe2x80x94O or Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O types; the Bi component may be partially substituted by Pb. There may be a plurality of superconductive high Tc phases within individual material systems, which phases differ through the number of copper-oxygen lattice planes or layers within the crystalline unit cell and have different critical temperatures.
It is desired for these known HTS materials to be deposited on different substrates for different applications, the aim generally being to form pure-phase superconductor material as far as possible. For example, metallic substrates are provided in particular for conductor applications.
Furthermore, the European specification mentioned above describes an oxidic superconductive shaped body with a substrate made from a polycrystalline metal or from ceramic, in which the substrate material is to have a coefficient of thermal expansion of between 5xc3x9710xe2x88x926/xc2x0 C. and 15xc3x9710xe2x88x926/xc2x0 C. If it is assumed that known HTS materials have coefficients of thermal expansion of the order of magnitude of approximately 10xc3x9710xe2x88x926/xc2x0 C., the expansion characteristic of the substrate is at least substantially matched to that of the HTS material. The substrate of the known shaped body is also covered with a layer of precious metal, for example of Au or Ag, which serves as a base for a buffer layer. This buffer layer consists of an inorganic material with a predetermined crystal structure and allows textured growth of the HTS material during a deposition process. Owing to the layer of precious metal on the substrate, the known shaped body cannot readily be used for a current limiter device. Since additional measures which reduce the switching capacity per unit surface area are required, for example in the form of a parallel connection of the precious metal layer with the HTS conductor, in order to prevent the current from sparking over from the HTS conductor via the buffer layer into the precious metal layer when an area of the HTS conductor becomes normally conductive.
Furthermore, German Specification 195 20 205 A describes the general use of electrically insulating substrates made from glass material as supports for conductor tracks made from HTS material in current limiter devices. To allow textured growth of the HTS material, in this case too a suitable buffer layer is applied to that surface of the substrate which is to be coated with the HTS material.
A further structure with a glass substrate as a support for an HTS layer is described in xe2x80x9cPhysica Cxe2x80x9d, Vol. 267, 1996, pages 335 to 360. To produce a biaxially oriented thin film from the HTS material YBa2Cu3O7xe2x88x92x, various glass substrates are selected from materials with a coefficient of thermal expansion xcex1 of at most 4.6xc3x9710xe2x88x926/xc2x0 C. Moreover, the substrates each had a very small surface area to be coated, which was covered with oriented, Y-stabilized ZrO2. However, it has emerged that with the known construction it is only possible to achieve critical current densities Jc of the order of magnitude of 104 A/cm2 (in the zero field).
Current densities of this level are regarded as being too low for many applications.
It is accordingly an object of the invention is to provide a construction with high Tc superconductor material and a process for producing the construction that overcome the above-mentioned disadvantages of the prior art constructions and processes of this general type, in such a way that higher critical current densities can be achieved compared to the literature reference mentioned above. Industrial manufacture, in particular using commercially available glass material is to be possible, in order for it to be possible to use the construction in current limiter devices with a large surface area.
With the foregoing and other objects in view there is provided, according to the invention, a metal-oxide high Tc superconductor material construction, comprising
an electrically insulating material substrate having a coefficient of thermal expansion which is matched to that of the superconductor material,
an intermediate layer bonded to the substrate so as to form a composite body with the substrate and comprising a glass material resistant to the maximum temperature occurring during the production of said construction and having a coefficient of thermal expansion greater than 6xc3x9710xe2x88x926 Kxe2x88x921,
at least one buffer layer which has been deposited on the intermediate layer, and
a layer of metal-oxide superconductor material which has been deposited on the buffer layer.
The invention is based on the recognition that the (linear) thermal expansion coefficient of the glass material together with its characteristic transformation temperature, which is of importance with regard to the maximum temperature required for the deposition or formation of the superconductor material, is the decisive variable with a view to obtaining a high critical current density Jc. If a glass material having the claimed level of expansion coefficient is provided, it is advantageously possible to at least substantially avoid cracking in the HTS material, a phenomenon which was observed in the superconductor structure described in the abovementioned literature reference xe2x80x9cPhysica Cxe2x80x9d, since this value is at least substantially matched to the value of the coefficient of expansion of the HTS material, which is of the order of magnitude of 10xc3x9710xe2x88x926 Kxe2x88x921. A glass material with a coefficient of expansion of over 7xc3x9710xe2x88x926 Kxe2x88x921is therefore to be considered particularly advantageous. The measurement range for the abovementioned values usually extends from 20xc2x0 C. to 300xc2x0 C.
Surprisingly, it has been observed that certain commercially available glass materials intended for other applications, possibly also with a sufficiently large surface area, have a sufficiently high transformation temperature, in particular higher than 500xc2x0 C., and also have the required high coefficient of expansion, even though commercially available glass materials with high coefficients of expansion generally do not have the required resistance to high temperatures such as those which are needed for the production of HTS materials.
Glass materials according to the invention which are suitable for the intermediate layer are relatively inexpensive, so that they can be used in particular for large-area substrates with a coatable area of at least 10 cm2, preferably over 100 cm2, as are to be provided in particular for current limiter devices according to the invention.
Specifically, in such devices a total surface area of HTS material of over 2 m2 is required for a power of, for example, approximately 10 MVA which is to be limited.
The use of an intermediate layer made from the glass material makes it possible, in a simple manner, to obtain a sufficiently smooth coatable surface of the composite body comprising substrate and intermediate layer.
Complex and expensive polishing processes to be carried out on the surface can thus be avoided.
The intermediate layer of glass material according to the invention can be formed by a glaze deposited on the substrate; alternatively, a plate or pane made from a heat-resistant flat glass having appropriate coefficient of thermal expansion is joined to the substrate, in particular by adhesive bonding.
With the foregoing and other objects in view, there is also provided, according to this invention, a process for producing a construction according to this invention comprising the steps of providing an electrically insulating material substrate having a coefficient of thermal expansion which is matched to that of the superconductor material, bonding to the substrate an intermediate layer comprising a glass material resistant to the maximum temperature occurring during the production of the construction and having a coefficient of thermal expansion greater than 6xc3x9710xe2x88x926 Kxe2x88x921, so as to form a composite body, depositing on the intermediate layer at least one buffer layer, and depositing on the at least one buffer layer a layer of metal-oxide superconductor material, wherein the maximum temperature on the substrate in at least one deposition process for the material of the buffer layer and/or the superconductor layer is held at most 150 K higher than the transformation temperature of the glass material.
Advantageously, the production of a construction according to the invention comprises a deposition process for the material of the buffer layer and/or the superconductor layer, in which the maximum temperature on the substrate is held preferably at most 100 K above the transformation temperature of the glass material. In this way, it is advantageously possible to avoid undesirable softening and expansion of the glass material, in particular when using intermediate layers with a large surface area.
Further advantageous configurations of the construction according to the invention and of the process for its production are given in the dependent claims.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a construction with high Tc superconductor material and process for producing the construction, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes can be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.